Capacitive deionization (CDI) cells are known for purifying or otherwise deionizing liquids such as water. For example, U.S. Pat. No. 5,954,937 discloses an electrically regeneratable electrochemical cell for capacitive deionization and electrochemical purification and regeneration of electrodes including two end plates, one at each end of the cell. Two end electrodes are arranged one at each end of the cell, adjacent to the end plates. An insulator layer is interposed between each end plate and the adjacent end electrode. Each end electrode includes a single sheet of conductive material having a high specific surface area and sorption capacity. In one embodiment of this disclosure, the sheet of conductive material is formed of carbon aerogel composite. The cell further includes a plurality of generally identical double-sided intermediate electrodes that are equidistally separated from each other, between the two end electrodes. As the electrolyte enters the cell, it flows through a continuous open serpentine channel defined by the electrodes, substantially parallel to the surfaces of the electrodes. By polarizing the cell, ions are removed from the electrolyte and are held in the electric double layers formed at the carbon aerogel surfaces of the electrodes. As the cell is saturated with the removed ions, the cell is regenerated electrically, thus minimizing secondary wastes.
U.S. Pat. No. 6,709,560 discloses flow-through capacitors that are provided with one or more charge barrier layers. Ions trapped in the pore volume of flow-through capacitors cause inefficiencies as these ions are expelled during the charge cycle into the purification path. A charge barrier layer holds these pore volume ions to one side of a desired flow stream, thereby increasing the efficiency with which the flow-through capacitor purifies or concentrates ions.
These references all produce useful CDI cells, but a CDI cell that performs better is still needed. The desirable CDI cell has a large capacitance to remove ions from a liquid stream, and is durable enough to be able to run over a longer period of time, has reduced scale buildup on the components such as the spacer, and a flatter pH curve.
As used herein, “effective capacitance” means dQ/dV for a membrane-electrode conjugate as determined by current interrupt as described herein.
Also as used herein, “durability” means hours until ion removal is less than 60% (under test conditions specified herein).